1. Field of the Invention
The present invention relates generally to magnetic resonance imaging. More specifically, the preferred embodiments of the present invention provide a magnetic resonance imaging method employing a fast spin echo technique in combination with hybrid radial-Cartesian image reconstruction. The present fast spin echo imaging technique of the preferred embodiments does not require the Carr Purcell Meiboom Gill (CPMG) condition. Among other things, the preferred embodiments of the present invention are particularly well suited for use with diffusion-weighted MRI imaging.
2. Background Discussion
Magnetic Resonance Imaging (MRI) is a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structure of objects having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR). In MRI, imposing a strong main magnetic field (B0) on the nuclei polarizes nuclei in the object to be imaged. The nuclei are excited by a radio frequency (RF) signal at characteristic NMR (Larmor) frequencies. By spatially distributing localized magnetic fields surrounding the object and analyzing the resulting RF responses from the nuclei, a map or image of these nuclei responses as a function of their spatial location is generated and displayed. An image of the nuclei responses provides a non-invasive view of an objects internal structure.
MRI machines are costly. Therefore, it is desirable to minimize the amount of scanning time required to create an image, while maintaining image quality (e.g. contrast, resolution and signal-to-noise ratio). So-called “fast spin echo” techniques are commonly used to minimize scan time while creating MRI images of acceptable quality. There exist a number of fast spin echo techniques. Fast spin echo imaging typically uses multiple spin echoes (an ‘echo train’) generated after a single excitation pulse.
In modern MRI methods, diffusion weighted imaging is commonly performed. In diffusion weighted imaging, the rate of diffusion of, e.g., water is measured in the imaged slice. Diffusion weighting improves the MRI image contrast, and provides additional useful information, as well known in the art.
Unfortunately, diffusion weighted imaging requires very powerful magnetic field gradient pulses which create difficulties when combined with fast spin echo imaging methods.
Specifically, fast spin echo methods are sensitive to the initial phase of the echo signal. The well-known Carr Purcell Meiboom Gill (CPMG) condition is generally required in order to perform fast spin echo imaging. Diffusion pulses disrupt the CPMG condition and distort the critical phase information, particularly when the diffusion pulses are applied before imaging. Consequently, diffusion weighted imaging is disruptive to fast spin echo imaging.
The CPMG condition is simple to implement: The radiofrequency (RF) phase of all refocusing RF pulses (180° pulses) need to be the same and at the same time 90° with respect to the RF phase of the excitation pulse (90° pulse). The first part of the diffusion gradient pulse pair is applied before the 180° pulses where remnants of unsettled gradients (eddy currents fields) and/or motion can cause unwanted phase errors, particularly between the excitation pulse and first refocusing pulse. This initial phase can be any value and introduces an error in the CPMG condition causing signal loss and imaging artifacts as was mentioned above.
The signal that arises in a fast spin echo train has a large number of contributions from signals generated on different so-called pathways. The number of signals can become large for later echoes and they both reinforce and cancel one another in the CPMG case. One of the effects caused by the loss of CPMG condition is that these signals no longer line up precisely in synch with each other. Consequently, the signals beat against each other and create image artifacts. One approach to removing these artifacts is to sacrifice half of the signal. This is done by tuning the diffusion gradient pulses such that the signals segregate into a pair of conditions. Then, one of the pair is ‘spoiled’, and the other is retained for use in generating the image. This has the advantage of making the signal very stable but at a high cost in signal strength. A MRI method that is insensitive to violation of the CPMG condition is described in SPLICE: Sub-Second Diffusion Sensitive MR Imaging Using a Modified Fast Spin Echo Acquisition Mode, by Fritz Schick in Magnetic Resonance in Medicine, 1997, Vol. 38, Pages 638-644. However, the method of Schick is limited to using a single echo train for image construction because the method requires phase consistency between echoes. Consequently, the method of Schick cannot produce high quality images.
Accordingly, there exists a need in the art of magnetic resonance imaging for, among other things, a technique that allows diffusion weighted imaging to be combined with fast spin echo imaging. The present inventors have determined that it would be particularly beneficial to provide a fast spin echo method that does not require the CPMG condition, as, e.g., such a method could allow fast spin echo techniques to be combined with diffusion imaging or diffusion weighting without problems of artifacts. Also, the present inventors have determined that it would be beneficial to provide a CPMG insensitive method that does not require phase consistency between echo trains. Also, the present inventors have determined that it would be an advance to provide a fast spin echo technique that does not require discarding ½ the echo signal.
Diffusion weighted imaging creates another problem, as well. The diffusion pulses are very powerful and produce macroscopic motion and mechanical vibrations in the MRI apparatus. Macroscopic motion degrades the image quality and creates artifacts. To obtain quality MRI images, the effect of macroscopic motion and vibrations must be reduced.
In the art, hybrid radial-Cartesian (HRC) image reconstruction techniques are known for generating MRI images utilizing k-space data. One such technique is known as “PROPELLER”, and is described in U.S. Pat. No. 6,882,148 to Pipe and Multishot Diffusion-Weighted FSE Using PROPELLER MRI, by Pipe J G, Farthing V G, Forbes K P in Magnetic Resonance in Medicine, 2002, vol. 47, pages 42-52, and Motion Correction With PROPELLER MRI, Application to Head Motion and Free Breathing Cardiac Imaging, by Pipe J G, Magnetic Resonance In Medicine, 1999, vol. 42, Pages 963-969. In the PROPELLER technique, each received echo train corresponds to a set of lines in k-space having a unique radial orientation. Imaging data acquisition is completed when k-space is filled with many radially-oriented lines.
Hybrid radial-Cartesian reconstruction techniques such as the PROPELLER method are effective for minimizing the effects of macroscopic motion and vibrations created by the diffusion pulses. Accordingly, HRC techniques can provide benefits when diffusion weighted imaging is being performed.
The present inventors have determined that it would be particularly beneficial to provide an imaging method that is robust when exposed to the combination of macroscopic motion, vibrations, phase disruption and destruction of the CPMG condition caused by diffusion pulses and that such a method could be widely used in MRI.